1. Field of Invention
The present invention relates to a mobile communication system, more particularly, to an improved method for transmitting a pilot strength measurement message (PSMM) in a mobile communication system.
2. Background of the Related Art
A handoff is a technique that when a mobile station in communicating moves from a current cell to a neighbor cell in a single communication system, the communication path is automatically changed and the communication maintains its continuity. The handoffs are classified into a soft handoff, a softer handoff and a hard handoff.
The soft handoff and the softer handoff make it possible that the mobile station maintains the continuity of the communication with a new base station (or sector) that uses the same frequency as a previous base station (or sector). The hard handoff is a technique that is used in the base stations having different frequencies or having different frame offsets and is used to make the communication without being interrupted when the mobile station moves in many base stations in which the soft handoff is not possible.
Of the handoff methods, the soft handoff is explained in detail by referring to FIG. 1.
FIG. 1 is a diagram showing the process that a mobile station establishes a base station according to a pilot strength threshold value.
As shown in FIG. 1, when a mobile station is between two base stations A and B and its pilot strength is on the upper threshold value (T_ADD), the mobile station makes the communication through the base station A. However, if the threshold strength is on the lower threshold value (T_DROP), the mobile station makes the communication through the base station B.
On the other hand, if the mobile station is in moving from the base station A to the base station B, the mobile station passes the handoff interval in which its communication is not interrupted. Then, the mobile station has to make the communication through both base station A and base station B. However, when the mobile station is on the lower threshold value (T_DROP), the communication channel of the mobile station through the base station A is disconnected and at the same time the communication of the mobile station is achieved only through the base station B.
FIG. 2 is a diagram showing the process in which the pilot strength measurement and retransmission are achieved according to the conventional method.
FIG. 2 shows the acknowledgment process of the base station for the PSMM according to the existing IS-95B and J-STD-008 standard rules.
As shown in FIG. 1, FIG. 2 shows the process in which when the mobile station selects one of the two neighbor base stations according to the pilot strength threshold value, the mobile station generates the PSMM and transmits the PSMM.
First, for a PSMM requesting an acknowledgment, the mobile station establishes a message sequence number (MSG_SEQ) and an acknowledgment required indicator (ACK_REQ) and transmits the PSMM to the corresponding base station through a reverse traffic channel. The mobile station stores the message sequence number (MSG_SEQ) corresponding to the PSMM requesting the acknowledgment in a MSG_SEQ_ACKs and stores acknowledgment status indicators (ACK_WAITINGs[n], 0xe2x89xa6nxe2x89xa67). If the logic value of ACK_WAITINGs[(MSG_SEQ_ACKs+4)mod8] is YES, the mobile station does not transmit a new message requesting a response. Thus, when a new PSMM is transmitted to the base station through the reverse traffic channel, the mobile station sets ACK_WAITINGs[MSG_SEQ_ACKs] of the PSMM with YES and stores them. After then, when the acknowledgment for the PSMM is received from the base station through the forward traffic channel, ACK_WAITINGs[ACK_SEQ] for the transmitted PSMM is set with NO. At this time, the base station makes a response by setting the value of ACK_SEQ field into MSG_SEQ of the received message and by transmitting it.
On the other hand, the total window index number of a mobile station defined in IS-95B and J-STD-008 is 8 (0-7). Among them, the simultaneously transmittable number of the acknowledgment requesting messages (PSMM) before a response for the first message among them is received and the number (n) of the ACK_WAITING[n] cannot exceed 4. If the number of ACK_WAITINGs[n] exceeds 4, the mobile station does not transmit an acknowledgment requesting new message (PSMM) to the base station. As aforementioned, when an acknowledgment is received from the base station through the forward communication channel, the value of ACK_WAITINGs[ACK_SEQ] of the PSMM is NO. When a new PSMM requesting an acknowledgment is transmitted to the base station, the value of ACK_WAITINGs[MSG_SEQ_ACKs] of the mobile station is set with YES.
After the mobile station transmits the acknowledgment requesting PSMM to the base station, the mobile station drives a timer for time (T1m). If the mobile station does not gain the acknowledgment from the base station until the time (T1m) is passed, the mobile station retransmits the PSMM that has the identical message sequence number (MSG_SEQ) to the base station. At this time, whenever the mobile station retransmits the corresponding PSMM, the number of retransmissions, that is a retry count number, (RETRY_COUNT Number) is stored. If the retry count number exceeds the maximum value (N1m), the mobile station declares the corresponding call drop.
When the mobile station moves, in order to establish a base station according to the pilot strength threshold value, the mobile station generates the first PSMM (F0) and then transmits the first PSMM to the base station. After transmitting the acknowledgment requesting first PSMM, the mobile station drives a timer for time (T1m). The time for this case is 400 ms.
If the mobile station does not gain the acknowledgment from the base station until the time (T1m) is passed, the mobile station retransmits the PSMM (R01) that has the identical message sequence number (MSG_SEQ) to the base station. The maximum number of the retransmissions is 13 as aforementioned.
When the mobile station keeps moving, in order to establish a new base station according to a new pilot strength threshold value, the mobile station generates a second PSMM (F1) and then transmits the acknowledgment requesting PSMM (F1) to the base station. After transmitting the acknowledgment requesting second PSMM (F1), the mobile station drives the timer for the time (T1m) and waits the acknowledgment of the base station. If the mobile station does not gain the acknowledgment from the base station until the time (T1m) is passed, the mobile station retransmits the PSMM (R11) that has the identical message sequence number (MSG_SEQ[1]) to the base station.
When the mobile station keeps moving in succession, in order to establish a new base station according to a new pilot strength threshold value, the mobile station may generate a third PSMM (F2). The third PSMM (F2) requests an acknowledgment and at the same time is transmitted to the base station. After transmitting the acknowledgment requesting third PSMM (F2) the mobile station drives the timer for the time (T1m=400 ms). If the mobile station does not gain the acknowledgment from the base station until the time (T1m=400 ms) is passed, the mobile station retransmits the PSMM (R21) that has the identical message sequence number (MSG_SEQ[2]) to the base station.
On the other hand, if the retransmitted PSMM (R01) that has the same message sequence number (MSG_SEQ[0]) as the acknowledgment requesting first PSMM (F0) does not gain an acknowledgment, a PSMM (R02) that has the identical message sequence number (MSG_SEQ[0]) is retransmitted to the base station.
After retransmitting the acknowledgment requesting third PSMM (R21) that has the same message sequence number (MSG_SEQ[2]), if the acknowledgement (A2) for the retransmitted PSMM (R21) is gained from the base station, the mobile station does not retransmit an acknowledgment requesting PSMM that has the same message sequence number (MSG_SEQ[2]) as the third PSMM (F2) to the base station any more.
However, as shown in FIG. 2, the first and second acknowledgment requesting PSMMs (F0, F1) are retransmitted by 5 and 3 times and then gain the acknowledgments (A0, A1), respectively. As the result, the retransmission of the PSMMs (R) having the same message sequence numbers as the PSMMs (F0, F1), respectively, is ceased.
As aforementioned, the mobile station has a limitation in retransmitting a PSMM. In other words the maximum retry count number (N1m) for retransmitting is limited. If a retry count number exceeds the maximum retry count number (N1m), the mobile station declares the call drop. Therefore, the mobile station fails in a handoff and returns to a call release.
On the other hand, whenever the mobile station exceeds the pilot strength threshold value, the PSMM is generated in succession. As far as each generated PSMM does not gain an acknowledgment from the base station, the PSMMs having the identical message sequence number (MSG_SEQ[n]) are retransmitted successively to the times of the maximum retry count number (N1m) to the base station.
The method described above is called a selective repeat scheme. This scheme makes the message exchange between a mobile station and a base station possible, but has the following problem.
According to the scheme, as described in FIG. 2, the PSMM may be retransmitted unnecessarily many times. In other words, whenever a new PSMM having a strength exceeding a threshold value (T_Add) is generated as the mobile station moves, the PSMM (R) is retransmitted until an acknowledgment is received from the base station, but the number of the retransmissions is limited to the maximum retry Count number (N1m). However, the most recently transmitted PSMM (for example, F2 and R21 in FIG. 2) is an effective message and all the previously transmitted PSMMs (F0, F1) are meaningless since the mobile station has been moved to a different place and therefore a new PSMM is generated.
Thus, the conventional PSMM transmitting method includes the unnecessary communication elements that a single PSMM is repeatedly retransmitted until an acknowledgment is gained.
Furthermore, under the current situation that the window size of the mobile station is 4, the ineffective utility of window may be caused.
The other problem is that the exact transmission is disturbed because of a continuous interference from the voice band signals.
The other problem is that the reliability is decreased since the call drop occurs when a retry count number exceeds the maximum retry count number (N1m).
Accordingly, the present invention is directed to provide an improved PSMM transmission method in a communication system, by which the old PSMMs are prohibited from being retransmitted unnecessarily.
Another object of the present invention is to provide an improved PSMM transmission method in a communication system which has a backward compatibility and the existing IS-95B and J-STD-008.
To achieve this and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the improved PSMM transmission method according to the present invention transmits the most recent PSMM among the PSMMs being successively generated to a base station and when the pre-generated other PSMMs manage the window indexes of the definite number of mobile stations and base stations or a new PSMM is generated, clears the states from the window indexes.
According to one embodiment of the present invention, the mobile station decides whether the generated message itself is the PSMM. If the message is the PSMM, then the mobile station decides whether the PSMM is the most recent PSMM. If the message is not the most recent one, the mobile station waits the most recent PSMM. If the message is the most recent one, to gain an acknowledgment, the mobile station transmits the most recent PSMM to the base station. If the mobile station does not gain the acknowledgment from the base station in response to the transmission, the mobile station retransmits the PSMM to the base station to the times of a maximum retry count number until the acknowledgment is gained. On the other hand, if the mobile station gains the acknowledgment from the base station in response to the transmission, the mobile station clears all the PSMMs including the most recent PSMM. Furthermore, in the present invention, the base station as well as the mobile station has the same window index and manages the PSMM transmission and acknowledgment. As aforementioned, according to the present invention, the unnecessary retransmission of the PSMMs is prohibited and therefrom the efficiency and the reliability of the communication system are improved.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.